A concave lens is kept in contact with a convex lens of focal length `20 cm`. The combination works as a converging lens of focal length `100 cm`. Calculate power of concave lens.

Options

A. `+2 D `
B. `- 2 D`
C. `+5 D`
D. `- 4 D`

Answer

D

Explanation

Here, `f_(1) = 20 cm, P_(1) = 100/(f_(1)) = 100/20 = 5D , F = 100 cm, P = 100/F = 100/100 = 1D`
As `P_(1) + P_(2) = P, P_(2) = P - P_(1) = 1 - 5 = -4 D`